3.1 Field of the Invention
The exemplary, illustrative technology described herein relates to Solid Oxide Fuel Cell (SOFC) systems and particularly to structural features and methods of making enclosure walls from thermally conductive materials and forming the enclosures to provide thermally conductive pathways designed to distribute thermal energy by thermal conduction in a desired manner. More specifically the technology relates to thermal energy management in an SOFC system by providing thermally conductive pathways configured to improve operating performance, safety, and reliability.
3.2 The Related Art
Conventional solid oxide fuel cell (SOFC) systems used to generate electrical energy by an electrochemical process that typically utilized gas to gas heat exchangers to transfer thermal energy from exhaust gases to incoming air. Example embodiments are disclosed in U.S. Pat. No. 8,557,451 entitled Fuel Processor for Fuel Cell System, issued Oct. 15, 2013 and in U.S. Pat. No. 8,197,976 entitled Solid Oxide Fuel Cell System with Hot Zones and Two-Stage Tail Gas Combustors. While the gas-to-gas heat exchangers transfer waste heat from exhaust gases to incoming cathode air, the overall systems operate with hot spots at the tail gas combustion chamber and other locations where fuel is being combusted.
In conventional SOFC systems, the temperature surrounding hot spots tends to exceed a safe operating temperature for many highly thermally conductive metals such as copper and aluminum. Additionally, highly thermally conductive metals such as copper and aluminum are often damaged by oxidation when exposed to oxygen rich cathode gasses used in conventional SOFC systems. This has led to reluctance in the art to use highly thermally conductive metals with conventional SOFC systems which are instead constructed with temperature resistant metals surrounding the hot spots to avoid burn through and other failures including shortened product life caused by metal oxidation. The temperature resistant metals tend to include high temperature super alloys usually comprising nickel and cobalt such as Hastelloy, Monel, Inconel, and others that are less likely to be damaged by prolonged high temperature and oxygen exposure. One problem with using high temperature super alloys is that they have a low coefficient of thermal conductivity such as less than about 40 W/m° K and more generally less than 20 W/m° K. As compared with more thermally conductive metals (e.g. aluminum and copper alloys) with a coefficient of thermal conductivity of more than 200 W/m° K, the high temperature super alloys are poorer thermal conductors. As a result heat transfer by thermal conduction in conventional SOFC enclosures is slow and the slow rate of thermal conduction tends to create permanent hot spots or temperature gradients in the overall structure of the SOFC system.
More recently SOFC systems have been constructed to promote thermal energy transfer by thermal conduction in order to reduce thermal gradients. One such system is disclosed in U.S. application Ser. No. 14/399,795 entitled SOFC-Conduction published Apr. 7, 2016 as US: 20160099476A1. This document discloses an SOFC system that is formed with inner and outer metal enclosures formed as thermally conductive pathways made from more thermally conductive metals such as aluminum and copper alloys in order to improve the rate of thermal conduction from hot spots to cooler areas of the structure. By providing a thermally conductive pathway with a higher coefficient of thermal conductivity and providing some walls with a larger thermal mass than others, thermal gradients are reduced as each different enclosure wall system tends to more rapidly normalize to a uniform temperature enabled by more rapid thermal conduction through the aluminum and copper enclosure walls which in some cases include a copper core.
Conventional SOFC systems utilize internal temperature sensors to measure instantaneous temperature at hot spot locations located inside hot zones of the SOFC system. An electric controller monitors the instantaneous temperature reported by each internal temperature sensor. If an over temperature condition is detected, the electronic controller is operable to shut down operation of the SOFC system by closing an input fuel valve. However, one problem with the use of internal sensors in a high temperature environment is that the thermal sensors can fail to provide the instantaneous internal temperature at all or can provide an inaccurate instantaneous temperature. As a result of a damaged or inaccurate internal thermal sensor an over temperature condition goes undetected and that can cause catastrophic failure such as a burn through of one of the enclosure walls. Other consequences include damage to thermal insulation surrounding hot zones and or damage to coating layers applied to internal and external enclosure wall surfaces. Even when damage due to an over temperature condition is minimal, when internal temperature sensors fail they need to be replaced. To replace a damaged internal temperature sensor the SOFC system must be disassembled and this is costly.
In view of the foregoing discussion there is a need in the art to provide an SOFC system that uses external temperature sensors to detect over temperature operating conductions that may lead to system damage or failure without relying on internal sensors. There is a further need in the art to provide a passive backup to thermal sensors that detect over temperature conditions. In particular the passive back is provided to shut down the SOFC system if the temperature sensor fails or otherwise reports an inaccurate temperature.